Two theoretical models of hepatic drug clearance are being studied with respect to the formation and elimination of drug metabolites by the liver. One of the models is the "well-stirred" model in which it is assumed that the unbound concentration of the drug in blood and in all hepatocytes is the same. The other model is the "parallel tube" model in which it is assumed that the unbound drug concentration declines exponentialy from the peripheral to the centrilobular region of the liver unit.